1. Field of the Invention
The present invention relates to a sheet stacking apparatus which stacks and aligns sheets on a stack tray and an image forming apparatus including this sheet stacking apparatus.
2. Description of the Related Art
Conventionally, as a sheet stacking apparatus that stacks and aligns a sheet with an formed image on a stack tray, a configuration in which a discharged sheet is aligned by an aligning member in a width direction perpendicular to a discharge direction of the sheet as disclosed in U.S. Patent Application Publication No. 2002/0079642 A1.
In the sheet stacking apparatus as disclosed in U.S. Patent Application Publication No. 2002/0079642 A1, the sheet discharged to the stack tray by a discharge roller is returned upstream in the discharge direction on the stack tray by a return portion and abutted to an abutment member to be aligned in a conveyance direction. Thereafter, the aligning member is operated in the width direction perpendicular to the discharge direction to abut an end surface of the sheet in the width direction to align the sheet in the width direction. Such an aligning operation is repeated every time the sheet is discharged to the stack tray.
As the sheet is discharged and stacked in sequence, the stack tray is sequentially lowered so that the height of a top surface of the stacked sheets is maintained constant. At this time, the height of the top surface of the stacked sheets is detected by a sheet surface detection member abutting the top surface of the sheet in the discharge direction at a position that is more upstream than the position where the aligning member abuts the end surface of the sheets stacked in the width direction. By such an operation, the stacked sheets are prevented from being pushed out or jammed by interference between an upstream end of the stacked sheet in the discharge direction and a downstream end of the sheet to be discharged subsequently in the discharge direction.
In the sheet stacking apparatus disclosed in U.S. Patent Application Publication No. 2002/0079642 A1, in the initial stacking state where the sheet is not stacked on the stack tray, an amount of intrusion of a sheet abutting portion of the aligning member into a sheet stack surface of the stack tray (an overlapping amount of the aligning member overlapped with the sheet) is secured. The above-described overlapping amount as an initial value is secured if the shape of the top surface of the sheets (or a sheet bundle) stacked on the stack tray is parallel to the sheet stack surface on the stack tray beveled so that a downstream portion thereof in the discharge direction is higher.
However, since the downstream end of the sheet in the discharge direction firstly lands on the stack tray and then the upstream end of the sheet in the discharge direction lands on the stack tray when the sheet curved to be convex (hereinafter, referred to as downward curl) is stacked one by one, for example, an air layer is formed between the sheets at an upstream side in the discharge direction. Therefore, as the amount of the stacked sheets increases, the top surface of the sheet becomes substantially horizontal at the downward side in the discharge direction and becomes relatively lower than the upstream side in the discharge direction with respect to the sheet stack surface on the stack tray which is beveled so that the downstream portion thereof in the discharge direction is high. That is, as the amount of the stacked sheets increases, the height of the top surface of the sheet at a position at which the sheet surface detection member abuts the top surface of the sheet in the discharge direction becomes relatively higher than the height of the top surface of the sheet at a position at which the aligning member abuts the sheet in the discharge direction. In this case, at the position at which the aligning member abuts the sheet in the discharge direction to align the sheet, as the amount of the stacked sheets increases, the overlapping amount of the aligning member and the sheet is not secured so that the alignment of the sheet cannot be eventually performed to remarkably impair the aligning property of the sheet on the stack tray.
Further, when the sheet curved to be concave (hereinafter, referred to as upward curl) is stacked, a similar problem is also occurred since the top surface of the sheet warps upward with respect to the sheet stack surface of the stack tray.